We pass in the world for sects and schools, for erudition and piety, and we are all the time ________ babes.

Inspite of attending the institutions for knowledge in this world, we are still like a child. This implies that the second part of the sentence, though beginning with and is contrary to the first part of the sentence. (Babes - a baby; infant provides the clue). Of all the options, “jejune” (not interesting or satisfying; dull or empty), is the most appropriate choice and replaces the blank. Hence, (3) is the answer.